Various feedback mixture control systems are known and used to clean exhaust gases from automotive internal combustion engines. Such a mixture control system typically uses an electric signal generated by an oxygen sensor adapted to detect and determine the concentration of oxygen in the exhaust gases from the engine. The electric signal produced by the oxygen sensor is processed in accordance with such schemes as to clean the exhaust gases from the engine and enables the control system to properly regulate the air-to-fuel ratio of the combustible mixture to be inducted into the power cylinders of the engine.
Among the various types of oxygen sensors used for such exhaust-gas cleaning purposes, there is an electrochemical oxygen sensor device using an oxygen-ion conductive solid electrolyte as an active material. Examples of a sensor device of this type are disclosed in U.S. Pat. No. 4,450,065 and Japanese Provisional Patent Publication No. 58-153155. The electrochemical oxygen sensor device shown in each of these is operative to generate a current or voltage output which varies in proportion to the detected concentration of oxygen.
Such an oxygen sensor unit largely consists of a combination of oxygen pump and sensor cell stacks or elements each comprising a pair of spaced electrode plates and an active layer of an oxygen-ion conductive solid electrolyte sandwiched between the electrode plates. The pump and cell stacks are spaced apart in parallel from each other to form between the respective inner electrode plates of the stacks a gap which is exposed to a passageway in, for example, the exhaust manifold of an internal combustion engine when the sensor device is in use. Typically, the electrode plates of the oxygen pump stack are subjected to a current so that the inner electrode plate is negative in polarity with respect to the outer electrode plate.
When a d.c. current is applied to the oxygen pump stack of the sensor device thus largely constructed, oxygen ions are caused to migrate through the oxygen-ion conductive active layer of the oxygen pump stack away from the negative inner electrode plate toward the positive outer electrode plate of the stack. As a result of such movement of oxygen ions through the active layer of the oxygen pump stack, the oxygen molecules contained in the exhaust gases in the gap between the pump and cell stacks are caused to diffuse into the active layer through the inner electrode plate, causing the same number of oxygen molecules to leave the active layer through the outer electrode plate of the oxygen pump stack. As the oxygen molecules are pumped out of the gap between the pump and cell stacks into the active layer of the oxygen pump stack, there accordingly is produced a gradual decrease in the concentration of oxygen in the gap so that a differential grows between the concentrations of oxygen in the gases in the gap and in the ambient gases in which the sensor unit is immersed within the exhaust manifold of the engine. Under a steady state condition which is then reached, a differential oxygen partial pressure is thus developed between the fluxes of the gases inside and outside the gap and acts on the oxygen-ion conductive active layer of the sensor cell stack. The sensor cell stack is accordingly caused to induce across the active layer thereof an electromotive force which varies with the differential between the pumped and non-pumped oxygen partial pressures. Such an electromotive force and accordingly the voltage (cell voltage) across the sensor cell stack varies with the concentration of the oxygen in the ambient exhaust gases if the current (pump current) applied to the oxygen pump stack is controlled to remain constant in order to maintain the concentration of the oxygen in the gap constant. If the cell voltage is maintained constant, the pump current varies widely. The pump current controlled to vary in this fashion is substantially proportional to the concentration of the oxygen in the ambient exhaust gases to which the oxygen sensor is subjected and can thus be used as a signal current in the aforesaid feedback mixture control system for an internal combustion engine.
If it happens that an excess of pump current is fed to the oxygen pump stack of the oxygen sensor unit, more oxygen molecules are pumped out of the oxygen-ion conductive active layer of the oxygen pump stack than those injected into the layer, which is accordingly deprived of an excess of oxygen molecules to create an oxygen-depleted state therein. This phenomenon is known as the "blackening" of the oxygen-ion conductive solid electrolyte used as the active material in the oxygen sensor unit. Where a solid electrolyte of, for example, zirconia (zirconium oxide, ZrO.sub.2) is used as the solid electrolytic substance, the zirconium component is separated from the active layer as a result of the blackening of the electrolytic substance. The blackening of the active layer of the oxygen pump stack causes accelerated deterioration of the pump stack and prevents the oxygen sensor unit from properly functioning.
With a view to precluding an occurrence of such blacking of the oxygen-ion conductive active layer of the oxygen pump stack, a feedback mixture control system using an oxygen sensor unit of the described nature is usually designed to limit the pump current below a critical value which varies with the concentration of oxygen detected by the sensor unit. The pump or activating current to be supplied to the oxygen pump stack is thus controlled to stay below such a critical value and is compared with a certain reference current to determine whether the air-to-fuel ratio of the mixture to be supplied to the power cylinders of the engine is to be shifted to the rich burn side or to the lean burn side. Such control of the air-to-fuel ratio of the mixture is usually effected by introducing secondary air through a secondary air introducing control valve. The secondary air introducing control valve is operated to shift the air-to-fuel ratio of the mixture to the lean burn side when made open and to shift the air-to-fuel ratio to the rich burn side when closed. The valve is operated to open and close depending upon the control signal produced on the basis of the pump current varied as above discussed. By the alternate opening and closing actions of such a secondary-air introducing control valve, the air-to-fuel ratio of the mixture to be supplied to the power cylinders of the engine is controlled toward a value represented by the reference current with which the pump current was compared.
FIG. 1 of the drawings is representative of the critical value of the pump current (I.sub.P) over which the oxygen-ion conductive active layer of the oxygen pump stack might cause the blackening when the oxygen sensor unit is placed in an atmosphere containing oxygen in a given concentration. As will be seen from this characteristic curve, the critical value of shown by the line d of FIG. 1 becomes the smaller and the pump current I.sub.p as the detected concentration of oxygen becomes smaller with the mixture supplied to the power cylinders of the engine made richer. When the mixture to be supplied to the power cylinders of the engine happens to be suddenly enriched for one reason or another (with, for example, the throttle valve in the induction system of the engine closed suddenly to deceleration), the secondary-air introducing control valve is actuated to open to supply additional air into the mixture immediately upon enrichment of the mixture. The pump current I.sub.p supplied to the oxygen sensor unit after the mixture has thus been leaned out subsequently to the sudden enrichment however increases beyond the critical value for the concentration of oxygen to result from the leaned mixture and would cause the blackening of the active layer of the oxygen pump stack. The pump current I.sub.p unchanged and is indicative of the oxygen concentration resulting from the enriched mixture for some time after the mixture is leaned out subsequently to the sudden enrichment. This is because of the fact that there inevitably exists a certain amount of time lag before the result of the combustion of the mixture diluted with the additional air is responded to by the oxygen sensor unit in the exhaust manifold after the mixture was leaned out in the induction system of the engine.
It is accordingly a prime object of the present invention to provide an improved oxygen concentration detection system having an electrochemical oxygen sensor unit which is provided with means to reliably protect the oxygen pump stack of the sensor unit from causing the blackening phenomenon which would otherwise be caused by an excess of pump current applied to the oxygen pump stack.
It is another important object of the present invention to provide an improved oxygen concentration detection system in which, when used in the exhaust system of an internal combustion engine, the oxygen sensor unit forming part of the system can be reliably protected from causing the blackening of the oxygen pump stack provided therein.